Since you're using NationBuilder as your email provider, there isn't an "inbox" in the traditional sense. Instead, we recommend creating a path to track your incoming email. 

Here is the general strategy:

1. Create a path called something like "Email"
2. Create steps like "Inbox" and "Needs followup"
3. Check the box for "Add people who email to a path step" in your broadcaster's settings, under Settings > Email (note that this checkbox will only appear after you've selected "Receive incoming email for this address").
4. Assign people who send emails to you to the path "Email," on the step "Inbox."

Now, when you go to check this path under the People tab in Paths view, all incoming emails will be slotted in your Inbox step. You can then close out the path (either by completing or abandoning it) if you don't need to respond, or slot it into your followup step so you can reach out to them later. When responding to an email, you can click on their profile directly from your path step, and scroll down on their Dashboard to find their email. 

For more information on how this can work, Chris Asmar wrote in detail on how he uses an email communication path to manage correspondence.